1. Field of the Invention
The present invention relates to an amplifier circuit for AM broadcasting, and more particularly, to one preferably applicable to an RF (Radio Frequency) amplifier used for a broadcast signal input stage of a radio receiver that receives AM broadcast signals, etc.
2. Description of the Related Art
FIG. 1 shows a configuration of a conventional AM broadcasting reception circuit. FIG. 1A shows a configuration of a tuning circuit format and FIG. 1B shows a configuration of a non-tuning circuit format. As shown in FIG. 1A, the conventional AM broadcasting reception circuit in the tuning circuit format is constructed of a capacitor 101, a resistor 102, an FET (Field Effect Transistor) for signal amplification 103, a tuning circuit 104 and an IC 106. Of these elements, the capacitor 101, resistor 102, FET for signal amplification 103 and tuning circuit 104 constitute an RF amplifier.
Here, the capacitor 101 is intended to cut a DC component of an AM broadcast signal inputted from an antenna which is not shown and the resistor 102 is intended to give an appropriate bias to the FET for signal amplification 103. The FET 103 for signal amplification is intended to amplify the inputted AM broadcast signal and is constructed of a junction FET (junction field effect transistor=JFET).
The tuning circuit 104 is intended to amplify an RF signal outputted from the FET for signal amplification 103 at a high frequency and output it to the IC 106 and constructed of a tuning capacitor C1 and tuning coils L1 and L2. One end of this tuning circuit 104 is connected to a power supply Vcc. On the other hand, the IC 106 is intended to input the RF amplified signal outputted from the tuning circuit 104 and perform subsequent signal processing necessary for AM broadcasting reception such as mixing and frequency conversion.
Furthermore, as shown in FIG. 1B, the AM broadcasting reception circuit in the non-tuning circuit format is constructed of a capacitor 101, a resistor 102, an FET for signal amplification 103, a coupling capacitor 105, an IC 106 and a coil 107. Of these elements, the capacitor 101, resistor 102, FET for signal amplification 103, coupling capacitor 105 and coil 107 constitute an RF amplifier.
The scale of integration of an RF circuit for a wireless terminal which handles high frequency signals of 2.4 GHz band or 5 GHz band, etc., is being increased in recent years and an LSI which incorporates an RF circuit which has been mounted outside a chip as an individual analog part so far on a single chip using a CMOS technology is under development. Likewise, an LSI incorporating an RF circuit using a CMOS technology for an FM broadcasting receiver using frequency bands of 76 M to 90 MHz is also under development. These RF circuits integrated on a single chip also include RF amplifiers.
On the other hand, as shown in FIG. 1, an AM broadcasting receiver uses a junction (bipolar) type JFET 103 for the RF amplifier and its manufacturing process is different from the CMOS technology, and therefore the AM broadcasting RF amplifier is still mounted outside the chip of the IC 106 as a separate component. This is because influences of flicker noise (1/f noise) produced inside a MOS semiconductor are taken into consideration.
That is, the noise level of flicker noise is inversely proportional to frequency, and therefore in the case of a wireless terminal handling a high frequency signal, almost no flicker noise is produced even if its RF amplifier is constructed of a CMOS circuit. However, in the case of an AM broadcasting receiver handling low frequency signals of medium frequency such as 530 to 1710 KHz or low frequency such as 153 to 279 KHz, their frequency bands still belong to areas with large flicker noise components, and therefore it is not desirable to construct an RF amplifier with a CMOS circuit.
For this reason, the JFET 103 has been conventionally used for the RF amplifier. Moreover, an RF amplifier constructed of the JFET 103 combined with a bipolar transistor has also been used. However, these conventional technologies are unable to integrate an RF amplifier together with another RF circuit, etc., on a single chip, and as a result, the conventional technologies have a problem that it is not possible to reduce the size of an entire circuit as in the case of a high frequency wireless terminal.
The present invention has been implemented to solve these problems and it is an object of the present invention to integrate an RF amplifier for AM broadcasting together with other circuits on one chip while suppressing the flicker noise to a lowest possible level and thereby realize small size and low noise of the entire circuit.